THE CIRCULATION OF THE BLOOD AND LYMPH 129 



proves that the heart can beat when separated from the central 

 nervous system, it does not prove that nervous influence is not 

 essential to its action, for in the cardiac substance nervous 

 elements, both cells and fibres, are to be found. 



The Intrinsic Nerves of the Heart. In the heart of the 

 frog numerous nerve-cells occur in the sinus venosus, especially 

 near its junction with the right auricle (Remak's ganglion). 

 A branch from each vagus, or rather from each vago-sympathetic 

 nerve (for in the frog the vagus is joined a little below its exit 

 from the skull by the sympathetic), enters the heart along the 

 superior vena cava (pp. 143, 182). 



Running through the sinus, with whose ganglion-cells the true 

 vagus fibres, or some of them, are believed to make physiological 

 junction (p. 149), the nerves pursue their course to the auricular 

 septum. Here they form an intricate plexus, studded with ganglion- 

 cells. From the plexus nerve-fibres issue in two main bundles, 

 which pass down the anterior and posterior borders of the septum 

 to end in two clumps of nerve-cells (Bidder's ganglia), situated at 

 the auriculo- ventricular groove. These ganglia in turn give off 

 fine nerve-bundles to the ventricle, which form three plexuses -one 

 under the pericardium, another under the endocardium, and a third 

 in the muscular wall itself, or myocardium. From the last of these 

 plexuses numerous non-medullated fibres run in among the muscular 

 fibres and end in close relation with them. Similar plexuses of 

 nerve-fibres exist in the mammalian ventricle. But while scattered 

 ganglion-cells are found in the upper part of the ventricular wall, 

 most observers have been unable to demonstrate any either in the 

 mammal or the frog in the apical half. In the rat's heart, accord- 

 ing to the careful observations of Schwartz, true ganglion-cells 

 are confined to an area on the posterior surface of the auricles, 

 lying always under the visceral pericardium. Other writers, how- 

 ever, have stated that ganglion-cells do exist in the apex both of 

 the cat's and of the frog's heart. In connection with the whole 

 question it must be borne in mind that in other organs improved 

 histological methods have brought typical nerve-cells to light in 

 situations where they were not suspected or were denied to exist, 

 and, further, that all investigators are not agreed upon the histological 

 criteria by which ganglion-cells are to be distinguished. 



Cause of the Rhythmical Beat of the Heart. Scarcely any 

 physiological question has excited greater interest for many 

 years than the mechanism of the heart-beat. Several properties 

 of the cardiac tissue ought to be distinguished in discussing this 

 question : (i) Its automatism i.e., its power of beating in the 

 absence of external stimuli ; (2) its rhythmicity i.e., its power of 

 responding to continuous stimulation by a series of rhythmically 

 repeated contractions ; (3) its conductivity i.e., its power of 

 conducting the contraction wave or the impulse to contraction 

 once it has been set up ; and (4) the power of co-ordination, in 

 virtue of which the various parts of the heart beat in a regular 

 sequence. 



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